Described below are an apparatus and a method for the contactless and nondestructive inspection of electrically conductive structures produced on or in a substrate or in a manner floating in a substrate.
The method relates, in particular, to the contactless and nondestructive inspection of electrically conductive structures on substrates extending in each case along an area. Corresponding substrates occur for example in so-called display backplanes or printed circuits. An electrical check of corresponding structures before the completion of the entire display is possible only to a limited extent. That applies primarily to structures having lengths in the mm or μm range or without direct contact with the edge of a substrate.
An electrically conductive structure is floating when it is completely enclosed in a substrate, has no direct contact with an edge or surface of the substrate and is electrically insulated.
One possibility for checking the electrical functionality is to use a known needle test. This involves pressing needles onto the corresponding conductive areas in the substrates and producing electrical connections. Afterward, it is possible to examine the structure between the needle contacts for electrical conductivity or with regard to the functionality. However, such a method has some disadvantages. Making contact without damage is not possible, or is possible only to a limited extent. Furthermore, the contact-making itself is likewise comparatively complex. In this regard, the needles have to be accurately aligned and ensure the contact-making with a defined pressure. In this case, the needles likewise experience mechanical wear. On account of this complex process, the inspection of many areas and thus an “inline” inspection can be efficiently implemented only to a limited extent.
A further known measuring method is optical inspection. The latter cannot make any statement about the electrical properties or about entirely satisfactory functionality. However, it is suitable for material inspection. It is thereby possible to check the lateral geometry of structures. Cracks in electrically conductive structures or short circuits in lower layers between deeper conductor tracks, which lead to faulty behavior, cannot, however, be reliably identified.
A further known solution is so-called contactless capacitive inspection. In this case, a capacitance dependent on the location is formed between the substrate surface and the sensor. Voltage and capacitance changes can subsequently be detected from this capacitive coupling. It is thereby possible to capacitively scan the surface of the substrate or to calculate applied voltages on the substrate surface. The substrates have to be correspondingly supplied with signals for corresponding measurements. In the case of capacitive surface scanning, the signal feed to the substrate can likewise be excited capacitively via an electrically conductive bearing area. On account of the capacitive coupling, the measurement signal reacts to changes in conductivity or capacitance and is therefore much better suited to the electrical inspection of corresponding structures than corresponding optical methods. FIG. 2 shows this known method. In this method, however, it is not possible to inspect very small structures which are not electrically connected, that is to say which are floating. Consequently, a signal cannot be directly applied to the structures. A capacitive coupling via the substrate receptacle is not possible in this case. For a measurable signal, the coupling area of the substrate receptacle has to be decisively greater than the coupling area with respect to the sensor, otherwise the difference between the measurement results “structure present” and “structure not present” cannot be measured.